The invention relates to a method of driving a laser diode which may be utilized in high rate data transmission or the like, for example, and a driving apparatus using the driving method.
FIG. 11 shows an example of a conventional laser diode driver arrangement in its simplest form. A pair of transistors Q.sub.1 and Q.sub.2 in differential connection have their emitters connected to a current source Isd in common. A fixed bias voltage Vb is connected to the base of one of the transistors, Q.sub.2. Accordingly, the transistors Q.sub.1 and Q.sub.2 operate as switching elements in a manner such that when one of them is turned on, the other is turned off. In the example shown, a laser diode LD is connected to the collector of the transistor Q.sub.1, the base of which is connected with an input terminal T.sub.IN which is fed with a pulse drive signal V.sub.IN of a positive polarity. Each time a pulse in the signal V.sub.IN rises to assume a high level, the transistor Q.sub.1. is turned on to cause a light emission from the laser diode LD, thus radiating radiation P.sub.OUT. When the pulse applied to the input terminal T.sub.IN falls, the transistors Q.sub.1 is turned off, while the transistor Q2 is turned on. The pair of transistors Q.sub.1 and Q.sub.2 are turned on and off in a differential manner to achieve a high rate switching operation.
In FIG. 12, there is illustrated a relationship between the current If passing through the laser diode LD and an output radiation P.sub.OUT, which are depicted at A and B, respectively. A flow of bias current Ib which is close to an emission initiating current Ith is maintained through the laser diode LD by a current source Isb. A sophistication is made such that as soon as the transistors Q.sub.1 is turned on, the drive current If immediately exceeds the emission initiating current Ith, thus minimizing a lag in the light emission. The drive current If is shown at A in FIG. 12 while a response of radiation P.sub.OUT is shown at B in FIG. 12. A lag of radiation P.sub.OUT with respect to the rising and the falling edge of the drive current If is shown exaggerated at T.sub.DLY(ON) and T.sub.DLY(OFF) as will be noted in the graph B of FIG. 12.
A lag in the termination of radiation P.sub.OUT is attributable to the fact that a carrier concentration which remains in the active layer of the laser diode LD fails to return to zero rapidly when the drive voltage is interrupted. A difficulty that is caused by the lag in reducing the carrier concentration is illustrated in FIG. 13
For example, as illustrated in FIG. 13A, as the OFF interval T.sub.1 of the drive current I.sup.f or the extinction interval becomes shortened, the drive current I.sub.f would begin to flow at a timing when the carrier concentration is not reduced sufficiently, as shown in FIG. 13B, the time interval T.sub.DLY(ON) until the next emission is reached would be shorter or T.sub.DLY(ON1) &gt;, T.sub.DLY(ON2), as illustrated in FIG. 13C. In the example shown in FIG. 2, it is longer again in the OFF interval T.sub.2, whereby the delay of the emission with respect to the immediately following rising edge of the drive current, or T.sub.DLY(ON3), is longer than T.sub.DLY(ON2). This means that a jitter is produced depending on the length of the extinction period T.sub.OFF, and the occurrence of a jitter presents a fault to the high rate transmission in a disadvantageous manner. This problem is discussed in"Effect of Bit-Rate, Bias and Threshold Currents on Turn-on Timing Jitter in Lasers Modulated with Uncoded and Coded Waveforms," IEEE Photonics Technology Letters, Vol. 8, No. 3, March 1996, pp 461-463. This literature indicates that the turn-on time T.sub.ON of the laser diode LD after extinction period T.sub.OFF is given as follows: ##EQU1##
where .tau. represents a residual carrier concentration (carrier lifetime) in the active layer of the laser diode LD. FIG. 14 shows the relationship as represented by the equation (1) where turn-on time T.sub.ON and the extinction period T.sub.OFF are normalized with .tau., respectively. Curves 12A, 12B and 12C depicted in FIG. 12 correspond to values of Im which are equal to 2Ith, 3Ith and 4Ith, respectively, for Ib=0.5 Ith. It will be seen from FIG. 14 that the turn-on time T.sub.ON varies with the extinction period T.sub.OFF for a range of T.sub.OFF from 0 to nearly three of four times .tau., but remains constant for any subsequent change in the extinction period T.sub.OFF. The lifetime .tau. of residual carriers of a laser diode is usually on the order of 2 to 3 ns, and thus it is seen that if a time interval from a preceding pulse is less than 10 ns, a jitter occurs unavoidably. However, the jitter itself (or T.sub.ON) can be reduced if the drive current If is increased.